The present invention relates to motor drives for electric motors and in particular to a surge protection circuit reducing erroneous false ground fault indications and providing improved component life.
Motor drives control the frequency and amplitude of the electrical power applied to an electrical motor to improve motor operation, for example, by improving motor starting and stopping, motor speed and torque control, motor synchronization, load management, and energy efficiency. For this purpose, the motor drive will typically receive three-phase line power and rectify it to produce a DC bus voltage. The DC bus voltage is then received by a set of switching semiconductor devices, typically operating in switching mode, to synthesize multiphase AC electrical power from the DC bus voltage. The frequency and amplitude of the synthesized power is controlled by controlling the switching of the semiconductor devices.
In order to comply with the standards of regulatory and certification agencies, motor drives must have over-voltage or “surge” protection on their connections to line power to protect the motor drive and its associated equipment from high voltage transients induced, for example, by lightning or the switching of inductive loads elsewhere on the line. The surge protection device of choice is a metal-oxide-varistor (MOV). As is understood in the art, an MOV provides a nonlinear resistor having a high resistance up to a threshold voltage at which point the resistance drops to provide for a substantially constant voltage regulation up to the power limits of the MOV device. MOV devices are typically constructed of sintered zinc-oxide with other metal oxide additives.
A set of MOVs may be used to provide a network protecting the motor drive from excessive voltages between any of the three phase line inputs (“line-to-line over voltage”) and between any of the line inputs and ground (“line-to-ground over voltage”).
One problem with MOV is that a small amount of current (“leakage current”) flows through these devices even when they are operating below their threshold voltage at normal line voltages. This leakage current, if flowing to ground, can trigger ground fault sensors used for fault detection particularly when several motor drives are connected to the same line inputs.
Further, the leakage current to the MOV degrades the MOV over time thus reducing its reliability.